1. Field of Invention
The present invention relates in general to diagnostics and performance analysis of electrical submersible pump (ESP) assemblies and, in particular, to an improved method and apparatus for monitoring the power system to an ESP assembly.
2. Background
Electrical submersible pump (ESP) assemblies are often used in hydrocarbon producing wells for enabling or improving the flow of fluids from within the well bore to the surface. A typical ESP assembly includes a centrifugal pump which is driven by a three-phase AC motor, both located in the well bore. A typical ESP assembly also includes a power source located at the surface, often comprising a variable speed drive (VSD) and an output transformer. Connecting the motor to the power source, i.e., the output transformer, is a cable. Because the motor can be a very long distance from the power source, e.g., the output transformer and variable speed drive, the voltage drop in the cable can be significant.
Moreover, in certain wells, flat rather than round cable can be used to save space and satisfy the available geometry. Flat cable is constructed with the three conductors in a line when viewed in cross section. Round cable is constructed with the three conductors in a triangular configuration when viewed in cross-section. Round cable can be substantially electrically symmetrical from phase to phase. That is, mutual inductance and capacitance can be substantially the same for phases A-B, B-C, and C-A in a round cable, as understood by those skilled in the art. This is not the case for flat cable construction. For long cable runs, and particularly for long cable runs of flat cable, phase imbalance can cause substantial voltage and current imbalances between the phases, leading to less then optimum performance.
The power system of typical ESP assembly can be divided into three main sections: the primary side, surface; the secondary side, surface; and the secondary side, downhole. Conventionally, only the first of these sections—the primary side, surface—is monitored. Characteristics for the remaining sections are inferred. For example, a variable speed drive will monitor and control the current and voltages in all three phases of the primary side. On, the secondary side, the current and voltages are inferred via the output transformer characteristics. Not all secondary side characteristics are detectable on the primary side, however. For example, a phase-to-ground short on the secondary side can lead to a large surface voltage imbalance on the secondary side, without the voltage imbalance being detectable on the primary side. Because the star point of an ESP motor is subsurface, there has been no convenient way of monitoring the out-of-balance voltage.
There is a commercial need for improvement in power systems for ESP assemblies. The oilfield ESP assembly is an expensive system, and failures are costly, both in terms of equipment replacement cost and also lost oil production. Any improvements that can assist in the prevention of failure or increase in the run life of the ESP assembly are of substantial economic benefit. Moreover, the energy costs of running ESP assemblies are substantial, and any system that can improve the overall electrical efficiency of the ESP assembly can reduce running costs.